Certain types of ink jet printers typically create a printed image by ejecting liquid ink droplets through orifices positioned on an orifice plate in a print head. In general, each ink jet of a multiple-orifice drop-on-demand ink jet array print head operates by displacing ink in an ink pressure chamber and ejecting ink droplets from an associated orifice. A driver mechanism is used to displace the ink in the ink pressure chamber. The driver mechanism typically includes a transducer (e.g., a piezo-ceramic material) bonded to a thin diaphragm. When a voltage is applied to the transducer, it displaces ink in the ink pressure chamber, causing the ink to flow through an inlet from an ink manifold to the ink pressure chamber and through an outlet and passageway to the orifice. From the orifice, the ink is jetted onto an image receiving medium, such as a print medium or an intermediate transfer drum that transfers the image to the print medium.
Two inks commonly used in ink jet printers are aqueous ink and phase change or solid ink. Phase change ink has a liquid phase when it is above its melting temperature, for example 86.degree. C., and a solid phase when it is at or below the melting temperature. Phase change ink is conveniently stored, transported and inserted into an ink jet printer assembly in solid phase. However, for phase change ink to be properly ejected from a print head, the ink must be in the liquid phase and relatively hot. Because it typically takes a few minutes for phase change ink to melt after heat has been applied to it, there must be a continuous supply of melted ink having the proper temperature for the print head to eject.
The chemistry of phase change inks poses challenges to providing a continual supply of phase change ink in the liquid state. It is generally undesirable to heat a large supply of phase change ink or to maintain phase change ink in a liquid state for extended periods of time because this results in "cooking" or degrading the ink. Therefore, heating of phase change inks must be carefully regulated to avoid such heat degradation, while simultaneously maintaining a sufficient supply of liquid ink to minimize delays and/or interruptions in printer operations to load ink.
Some ink jet printers utilize a print head that translates or shuttles bi-directionally relative to a transfer drum or a final receiving medium. Many of these printers also incorporate a stationary ink supply assembly that requires the print head to be positioned adjacent to the supply assembly for delivery of ink to the print head. In order to minimize printing interruptions for ink loading, these systems must have a relatively large amount of liquid ink available to the print head at all times. In the case of phase change ink, this requires the ink to be kept in a molten state at an elevated temperature for extended periods of time, thereby creating the possibility of ink degradation.
It is known in the prior art to utilize a reservoir for maintaining a supply of liquid ink and a detector for determining whether the ink in the reservoir is below a particular level. Typically, the ink level detector indicates a low ink condition to the operator and/or triggers an automatic ink loading procedure only when the ink level drops below a critical fixed amount. This can result in interruptions and delays in printer operations for ink loading, as well as a large amount of ink remaining in the molten state for extended periods of time.
To overcome the drawbacks of the prior art systems, an improved method for providing a continuous supply of liquid ink to an ink jet print head is provided. The method coordinates the supply of liquid ink with other printer operations to minimize ink loading interventions. The method also ensures that the print head is consistently supplied with an adequate amount of molten ink to minimize interruptions and/or delays for ink loading. This is accomplished by maintaining a continuously updated amount estimate for each reservoir in a memory source and comparing the estimate to a plurality of ranges to determine if a reservoir should receive additional ink.